This invention concerns enhanced oil recovery processes which involve the displacement of oil by the injection of a gas or dense fluid through an injection well in order to produce oil at a production well. More particularly, this invention concerns improving the sweep of the recovery process by the generation of foam. The utility of the invention lies in the improvement in sweep efficiency when used in enhanced oil recovery processes. Sweep efficiency is broadly defined as volume of formation swept/total volume.
The use of a gas, a dense fluid, or a mixture of gases to displace and recovery oil from subterranean formations is well known. For example, U.S. Pat. No. 2,866,507 teaches a method for improving oil recovery by gas drive through the injection of an aqueous slug containing a surfactant having foam-producing characteristics. The aqueous slug is immediately followed by the gas drive. U.S. Pat. No. 3,185,634 teaches improved oil recovery by the simultaneous injection of a gas and a liquid containing a foaming agent. U.S. Pat. No. 3,318,379 teaches a method of improving oil recovery by introducing into the reservoir a slug containing a foaming agent, followed by a non-aqueous and surfactant-free slug, followed by a gas slug. U.S. Pat. No. 3,344,857 teaches a method for oil recovery by injecting carbon dioxide between two aqueous slugs. Each of the aqueous slugs contains a surfactant. U.S. Pat. No. 3,376,924 teaches a method for oil recovery by the injection of a surfactant slug, followed by a gas drive, forming a foam bank. After further oil production, the production well is shut in, and gas is injected until the formation pressure at the production well approximately equals the injection well pressure. Then the gas injection is stopped, and production is resumed. This method offers a spring-like compression and expansion on the foam.
The use of foam to improve the sweep efficiency of the displacing fluid involves the utilization of two foam properties. The first is the high resistance to flow that is associated with foam. The second property is the high gas-liquid surface area. Thus, only relatively small amounts of an aqueous solution of a foaming agent need be used with relatively large amounts of gas or dense fluid. The gas disperses in the liquid, generating a large interfacial area and a large volume of foam, thereby increasing the resistance to flow. If this resistance to flow is in those regions of the reservoir where the resistance is least, then the displacing fluid is forced to flow through regions of higher resistance, sweeping larger portions of the reservoir and recovering larger quantities of oil. Thus, the use of foam improves sweep efficiency.
In spite of the large gas-liquid surface area which is associated with foam, large quantities of a foaming agent are required and may be attributed to one or more of the following: large pore volume of the reservoir, losses due to adsorption on rock surface, partitioning to oil, and decomposition at reservoir conditions. The result is high cost. In many instances, this cost becomes so high that the process is rendered economically unattractive.